GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 33 



Figure 53 shows a ventral view of the epitheUoid bodies of 

 a thjToidless larva of the same age as the control, but of nearly 

 twice the volume. These bodies are actually about three times 

 as large as those of the control larvae, and relatively they are 

 somewhat larger than those of the controls. 



In still older and larger thyroidless larvae the epithelioid 

 bodies are relatively larger than in those just described. Their 

 volume is not easily measured, however, because they become 

 flattened somewhat and the formula for measuring the volume 

 of a sphere is no longer applicable to them, although they might 

 be measured as ellipsoids. 



Microscopic structure. Since the epithelioid bodies are be- 

 lieved to correspond to the parathyroid glands of higher animals, 

 it would not be unlikely that they should undergo some modi- 

 fication after removal of the thyroid, but no structural change 

 is evident in our material (figs. 68 and 69). The epithelioid 

 bodies of control and thyroidless larvae shown here are seen 

 to be composed of cells with large darkly staining round or oval 

 nuclei and finely granular cytoplasm. The cell boundaries are 

 almost indistinguishable. The body has a well-defined capsule. 

 Its blood supply is poor and this together with its cytological 

 appearance argues against any great secretory activity on its 

 part. After metamorphosis and in our older thyroidless larvae 

 the structure of these bodies seems to be the same as in the 

 normal larvae. 



Thyroid. The thyroid is missing from all the experimental 

 larvae which did not metamorphose, but it was studied in the 

 normal tadpoles and young frogs. Normally, there are two 

 thyroids, located on the ventral side of the hyoid at its posterior 

 border near the median plane. We found one specimen in which 

 the two thyroids were connected by an isthmus of thyroid tissue 

 as in mammals. In another specimen (fig. 54) the two were 

 nearly joined together. Comparison of the thyroid of twenty 

 control larvae and twenty-five young frogs shows that the vol- 

 ume is practically unchanged during metamorphosis (figs. 54 

 and 55) . This is an important point, since removal of the thyroid 

 prevents the occurrence of this transformation. Neither the 



